Alpine

apk command
$ apk update
$ apk upgrade

$ apk add openssh --no-cache
$ apk add openrc --no-cache

Add user|group
$ addgroup -S mygroup 
$ adduser -S myuser -G mygroup 
docker all future commands should run as the myuser user
$ USER myuser 

How do I add a user when I'm using Alpine as a base image?

The flags for adduser are:
Usage: adduser [OPTIONS] USER [GROUP]
Create new user, or add USER to GROUP
-h DIR          Home directory
-g GECOS        GECOS field
-s SHELL        Login shell
-G GRP          Group
-S              Create a system user
-D              Don't assign a password
-H              Don't create home directory
-u UID          User id
-k SKEL         Skeleton directory (/etc/skel)

$ rc-status --list

$ touch /run/openrc/softlevel